An audio distribution system consists of a set of audio inputs connected in such a way as to either mix or route these inputs to a single output or set of outputs. There is always one output that may have a multiplicity of inputs which will be either selectable or mixed together to form the composite audio signal, which, after amplification, is fed to output device(s), usually loudspeakers.
The conventional way of combining and routing all these input signals is via a mixer, In the mixer, the audio inputs are combined at various selected levels using volume controls to give a composite output signal which may be fed to an amplifier for amplification and hence to output devices, for example, loudspeakers. Some common features of these mixers are:
Where many inputs are required the mixer needs to be able to handle and control many different inputs which may be in different signal formats, for example, from a microphone and from the stereo output of for example, a CD player.
Many different audio products have different audio output connections as standard. These connectors are not suited to a single ‘mixer’ which tends to have many audio inputs all with the same input connection system.
All of the audio inputs have to be cabled back to the same point, that is the composite mixer position.
In typical systems different inputs at different input levels are required for, for example background music (BGM) for a stereo output at 0 db (relative to 1vRMS-Line Level) requiring its own mixing, and a microphone input from an electret microphone at −60 db typically.
These conventional types of mixers are used in most audio applications where the audio inputs are all relatively near to each other and prove convenient. However, currently there is an increasing demand for systems to have many inputs spatially distributed around a building at different points with a requirement for many different audio connectors which are not compatible with conventional mixers. In these types of applications conventional mixing systems have several disadvantages:
There will be long cable runs to the mixer from some inputs from various points in a building. If these cables carry conventional unbalanced microphone level inputs then they will be very susceptible to electromagnetic pick up and interference characterized normally by ‘mains hum’.
Quite often only one or two inputs are required and the mixers tend to be a least 6-way type units. This leads to many redundant and unnecessary inputs.
Different plug and connection systems employed now in the audio industry (both industrial and domestic) mean that virtually every conventional mixer will require at least some form of special lead to be manufactured from the input device to the mixer and in the case of a stereo signal will require that two inputs are used to mix the stereo signal. Even where the connection system is compatible, the cable from the audio device needs to be so long that it requires extending quite often in very expensive shielded cable.
Many audio sources are not of the so called ‘balanced type’, In this type of source there are two wires in which neither is grounded. The audio signal is defined as the difference between the signal levels on the two wires. Thus any common mode induced signal is eliminated when the signal is terminated by a suitable balanced to unbalanced converter. This means that if they are not of a high signal level they are susceptible to electromagnetic induction (EMI) causing interference with the wanted signal.
Clearly in situations where audio sources are to be sited spatially away from the central mixing equipment this situation is not ideal. It would be beneficial if the signal conditioning were to be effected nearer to the audio source together with the provision of a suitable connection system to suit the audio source.